How do I reconcile 98.4% from Medutest with 96.8% from the supplier?

What I am working with: 98.4% · Medutest · 96.8%.

I noticed this today and I have not touched anything since, in case the state is diagnostic.

I have not discarded anything yet, so a test is still possible if that is the recommendation.

Is this recoverable, and how would I tell?

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

Stated carefully, the fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

To be exact about it, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.

Compare purity within a single laboratory on the same method, never across laboratories.

Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

Gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.